With concern about the environment, considerable developments have been recently made in electric automobiles and solar-electric power generation that produce less environmental pollution. A direct current of several kilowatts to tens of kilowatts is dealt with in an electric car or solar-electric power generation. Therefore, a non-contact current sensor apparatus is required for measuring a direct current of tens to hundreds of amperes. Since the demand for such current sensor apparatuses is extremely great, it is requested in society to provide current sensor apparatuses that are inexpensive and exhibit high accuracy.
A current sensor apparatus incorporating a Hall element as a magnetic sensor is widely used for non-contact measurement of an electric current through measuring a magnetic field generated by the current with the magnetic sensor.
However, the Hall element has a problem of offset voltage that requires troublesome handling, which prevents a reduction in the price of the current sensor apparatus. The offset voltage means a residual output voltage when the magnetic field to be measured is zero.
There is a magnetic sensor apparatus or a current sensor apparatus that incorporates a fluxgate element as a magnetic sensor that utilizes saturation of a magnetic core. Attention has been given to such an apparatus that is expected to produce no offset voltage, according to the principle.
Reference is now made to FIG. 12 to describe the operation principle of a fluxgate element having the simplest configuration. FIG. 12 is a plot for showing the relationship between an inductance of a coil wound around a magnetic core and a coil current. Since the core has a magnetic saturation property, the effective permeability of the core is reduced and the inductance of the coil is reduced if the coil current increases. Therefore, if bias magnetic field B is applied to the core by a magnet and the like, the magnitude of external magnetic field H.sub.0 is measured as a change in inductance of the coil when external field H.sub.0 is superposed on the bias field. This is the operation principle of the simplest fluxgate element. In FIG. 12 each of bias field B and external field H.sub.0 is expressed in the magnitude converted to the coil current.
In this method the position of bias point B changes with factors such as the intensity of the magnetic field generated by the magnet or the positions of the magnet and the core in relation to each other. It is therefore required to maintain the inductance at a specific value when the external magnetic field is zero. However, it is extremely difficult to compensate for the instability of the inductance value due to temperature changes and other external perturbations. This method is therefore not suitable for practical applications.
If a rod-shaped magnetic core is used, an open magnetic circuit is provided, so that the effect of hysteresis is generally very small. Assuming that the hysteresis of the core is negligible, the characteristic of variations in inductance is equal when the coil current flows in the positive direction and in the negative direction since the saturation characteristic of the core is independent of the direction of coil current. For example, it is assumed that point P.sub.+ and point P.sub.- of FIG. 12 represent the coil current in the positive direction and the coil current of the negative direction, respectively, whose absolute values are equal to each other. In the neighborhood of each of these points, the characteristic of variations in inductance with respect to variations in the absolute value of the coil current is equal. Therefore, an alternating current may be applied to the coil such that the core is driven into a saturation region at a peak, and the difference in the amounts of decreases in the inductance may be measured when positive and negative peak values of the current are obtained. As a result, the difference thus measured is constantly zero when the external magnetic field is zero, which is always the case even when the characteristics of the core change due to temperature changes or external perturbations. That is, no offset voltage is generated in this case. In the present patent application a saturation region of the magnetic core means a region where an absolute value of the magnetic field is greater than the absolute value of the magnetic field when the permeability of the core is maximum.
An external magnetic field is assumed to be applied to the core. If external field H.sub.0 is applied in the positive direction of the current, as shown in FIG. 12, for example, the inductance value decreases at the positive peak of the current (point Q.sub.+ in FIG. 12, for example) and the inductance value increases at the negative peak of the current (point Q.sub.- in FIG. 12, for example). Therefore, the difference between these values is other than zero. Since the difference in the inductance values depends on the external magnetic field, the external field is obtained by measuring the difference in the inductance values.
With regard to a magnetic sensor apparatus or a current sensor apparatus incorporating a fluxgate element, the difference of the inductance values described above may be obtained from a signal obtained through differentiating the voltage generated across another inductance element connected in series to the sensor coil, that is, a signal equivalent to the second-order differential coefficient of the current flowing through the sensor coil.
The method thus described is called a large amplitude excitation method in the present patent application, that is, to apply an alternating current to the sensor coil such that the core is driven into a saturation region at a peak, and to measure the difference in the amounts of decreases in inductance at positive and negative peak values of the current.
In Published Unexamined Japanese Patent Application Hei 4-24574 (1992), an oscillation circuit including a resonant circuit part of which is made up of a sensor coil is disclosed. The oscillation circuit is provided as a means for applying an alternating current to the sensor coil.
When an external magnetic field is zero, it is required that the excitation current of the sensor coil has a wave with symmetrical positive and negative portions in order that the difference between the inductance values of the sensor coil at the positive and negative peaks of the current is zero.
However, the positive and negative portions of the waveform of the excitation current are not symmetrical, strictly speaking, if a drive circuit for exciting the sensor coil is actually fabricated and its operation is studied in detail. If a self-excited oscillation circuit is used as the drive circuit, in particular, asymmetry between the positive and negative portions of the wave of the excitation current is considerably great. Therefore, an offset voltage that is not negligible is generated in practice by a sensor apparatus utilizing the large amplitude excitation method, too.
The problems resulting from the offset voltage are that: the offset voltage causes a constant error in the output of the sensor apparatus; and that the offset voltage varies due to external perturbations such as a temperature and supply voltage.
It is known through observation that it is energy loss in the control input of an active element making up the oscillation circuit that induces the asymmetry between the positive and negative portions of the wave of the excitation current. It is also known that the major one of the external perturbations that cause variations in the asymmetry mentioned above is variations in the operating temperature of the active element making up the oscillation circuit.
Reference is now made to FIG. 13 to FIG. 15 to describe in detail the asymmetry between the positive and negative portions of the wave of the excitation current mentioned above.
FIG. 13 is a block diagram illustrating an example of the configuration of a magnetic sensor apparatus incorporating a fluxgate element. This magnetic sensor apparatus comprises: a magnetic core 201; a sensor coil 202 made up of at least one coil wound around the core 201; an alternating current supply section 203 for supplying an alternating drive current to the sensor coil 202 such that the core 201 is driven into a saturation region, an end of the section 203 being connected to an end of the sensor coil 202, the other end of the section 203 being grounded; and an inductance element 204 for detecting variations in the inductance value of the sensor coil 202, the element 204 being connected to the sensor coil 202 in series. The inductance element 204 has an end connected to the other end of the sensor coil 202 and the other end grounded.
The magnetic sensor apparatus shown in FIG. 13 further comprises: a differentiation circuit 205 for differentiating the voltage generated across the inductance element 204, the circuit 205 being connected to the node between the sensor coil 202 and the inductance element 204; a positive peak hold circuit 206 for holding a positive peak value of an output signal of the differentiation circuit 205; a negative peak hold circuit 207 for holding a negative peak value of the output signal of the differentiation circuit 205; an adding circuit 208 for adding the value held at the positive peak hold circuit 206 to the value held at the negative peak hold circuit 207; and an output terminal 209 from which an output signal of the adding circuit 208 is outputted.
In the magnetic sensor apparatus shown in FIG. 13, the alternating current supply section 203 supplies an excitation current of the sensor coil 202. This excitation current is differentiated twice at the inductance element 204 and the differentiation circuit 205, and made into a spike-shaped voltage signal that contains voltage values having opposite polarities and indicates positive and negative peak values of the excitation current. Each of these peak values of the positive and negative spike-shaped voltage values of this signal is held at the positive peak hold circuit 206 and the negative peak hold circuit 207, respectively, and added to each other at the adding circuit 208. The result is then outputted from the terminal 209 as an output signal.
In the magnetic sensor apparatus shown in FIG. 13, the output signal is zero and no offset voltage is generated if the positive and negative portions of the wave of the excitation current of the sensor coil 202 are symmetrical, and the external magnetic field applied to the sensor coil 202 is zero.
However, as described above, the positive and negative portions of the wave of the excitation current are not symmetrical, strictly speaking, if a drive circuit is actually fabricated and studied in detail. If a self-excited oscillation circuit is used as the drive circuit, in particular, asymmetry between the positive and negative portions of the wave of the excitation current is considerably great. Therefore, an offset voltage that is not negligible is generated.
Reference is now made to FIG. 14 to describe the cause of asymmetry between positive and negative portions of the wave of the excitation current when a self-excited oscillation circuit is used. The following is description of an example wherein a Clapp oscillation circuit incorporating a bipolar transistor as an active element is used as the self-excited oscillation circuit. FIG. 14 is a circuit diagram illustrating an example of the configuration of the Clapp oscillation circuit for exciting the sensor coil 202.
The Clapp oscillation circuit shown in FIG. 14 comprises: an npn bipolar transistor 211; the sensor coil 202 that also functions as a resonant coil; and a capacitor 212 for resonance connected in series to the sensor coil 202. The sensor coil 202 and the capacitor 212 make up a series resonant circuit. The base of the transistor 211 is connected to an end of the sensor coil 202 through the capacitor 212. The other end of the sensor coil 202 is grounded. An end of a feedback capacitor 213 is connected to the base of the transistor 211. An end of a feedback capacitor 214 and the emitter of the transistor 211 are connected to the other end of the feedback capacitor 213. The other end of the capacitor 214 is grounded. The emitter of the transistor 211 is grounded through an emitter load coil 215. The collector of the transistor 211 is connected to a power input 216 and to the base through a bias resistor 217.
Consideration will now be given to an oscillation wave observed at the base of the transistor 211 in the oscillation circuit shown in FIG. 14. A base current is supplied to the transistor 211 in the neighborhood of the positive peak value of the oscillation wave. The transistor 211 then turns on and the capacitor 214 is charged by the emitter current. The energy produced through this charging is used for continuation of oscillation. It is noted that the base current of the transistor 211 does not flow in the neighborhood of the negative peak value of the oscillation wave, but only flows in the neighborhood of the positive peak value. As a result, part of the resonant energy is consumed as a base current only in the neighborhood of the positive peak value of the oscillation wave. In addition, if the transistor 211 is saturated, the combination of the base and the emitter is simply an equivalent of a diode. Therefore, the oscillation wave observed at the base of the transistor 211 has a shape in which a portion near a positive peak value is clamped, as shown in FIG. 15. In this way, asymmetry of the oscillation wave between positive and negative portions is created, that is, asymmetry of the wave of the excitation current between positive and negative portions is created. In FIG. 15 V.sub.CL indicates a clamping potential.
If value Q of the resonant circuit is sufficiently great, the asymmetry of the oscillation wave is corrected by the resonant circuit. However, value Q of the sensor coil 102 is not very great with regard to the magnetic sensor incorporating the fluxgate element. Therefore, the asymmetry mentioned above remains, which causes generation of an offset voltage.
Moreover, in the oscillation circuit shown in FIG. 14, the forward stopping potential between the base and emitter of the transistor 211 decreases as the operating temperature of the transistor 211 rises. The clamping potential thereby decreases. That is, the asymmetry mentioned above becomes greater as the operating temperature of the transistor 211 increases, and the offset voltage increases.
In the oscillation circuit shown in FIG. 14 the oscillation amplitude increases as the operating temperature of the transistor 211 rises, which promotes an increase in the offset voltage. This fact will be described as follows.
Since the excitation current supplied from the oscillation circuit contains no direct current components, the area of the positive portion and the area of the negative portion of the excitation current waveform are equal. If the excitation current waveform has asymmetrical positive and negative portions, the amount of an increase in amplitude in the positive portion is not equal to the amount of an increase in amplitude in the negative portion when the amplitude of the entire wave is increased.
For example, a case is assumed for convenience, in which the positive portion of the excitation current waveform is a trapezoid, the negative portion is a triangle, and each of the trapezoid and the triangle has a base equal in length. If the amplitude of the entire wave of the excitation current is increased, the amount of an increase in amplitude is greater in the negative portion than in the positive portion since the area of the positive portion is equal to that of the negative portion. In such a manner, with regard to the oscillation circuit shown in FIG. 14, the oscillation amplitude increases as the operating temperature of the transistor 211 rises. As a result, the asymmetry of the excitation current wave with respect to the positive and negative portions becomes greater and the offset voltage increases.
In order to solve the foregoing problems, with regard to the example of the oscillation circuit shown in FIG. 14, it is required to: minimize the energy consumed as a base current in the neighborhood of a positive peak value of the oscillation wave; reduce clamping between the base and the emitter; and eliminate variations in the clamping potential due to a temperature and variations in the oscillation amplitude due to a temperature.
The foregoing problems will not be solved only by adopting a Darlington transistor, a junction field-effect transistor (a field-effect transistor may be called a FET in the following description), or a metal-oxide semiconductor (MOS) FET. The reason will now be described.
In an actual experiment, it is possible to reduce energy consumed as a base current when a Darlington transistor is utilized. However, the equivalent current amplification factor is extremely increased. Operation instability due to the dependence of collector current leakage on a temperature is thus increased. Adopting the Darlington transistor is therefore not practical. If a junction FET is adopted, clamping of the oscillation wave is impossible since the FET is voltage-controlled. However, there is no type of junction FET that has a large current-carrying capacity. If the junction FET is incorporated in an excitation circuit that requires a large excitation current, power loss due to the internal resistance of the junction FET is so great that it is not suitable for practical applications. If a MOS FET is utilized, the MOS FET has a drawback similar to that of the junction FET if the MOS FET is an element having a high on-state resistance. If the MOS FET has a low on-state resistance, distortion of the oscillation wave due to the gate-source capacitance is great. The drawback thereof is therefore greater than that of a bipolar transistor.
As described so far, the following three points are important for solving the problems of offset voltage of the magnetic sensor apparatus or current sensor apparatus incorporating a fluxgate element.
(1) To minimize asymmetry of the oscillation wave between positive and negative portions.
(2) To prevent variations in asymmetry of the oscillation wave with respect to positive and negative portions, due to the operating temperature of an active element of the oscillation circuit.
(3) To prevent variations in oscillation amplitude, due to the operating temperature of an active element of the oscillation circuit.
Point (1) reduces the absolute value of the offset voltage. Point (2) eliminates variations in offset voltage due to temperature changes. Point (3) prevents an increase in offset voltage due to temperature changes.